I am looking for pointers on the protocol used for the communication between the main 82143 unit and the module that plugs into the HP-41 port (essentially the flat end of the 82143 cable that contains the HP-41 ROM and printer interface logic). I.e. I want to know what is going on in the wire that runs between the printer with the HP-41.

Why I want this info: To use the HP-41 module to talk to a different printer, or to interface the printing mechanism of the 82143 to a different computer.

I don't know the exact details (one day I must figure them all out), but I have a little general information that might be useful.
The 82143 'module' contains 2 chips, both direct-on-board. One is the ROM containing the HP41 Mcode to drive the printer. The other is called NPIC (Nut Peripheral Interface Chip), I believe. It's basically a pair of 56 bit shift registers, one for data from the HP41 to the 82143, one for data from the 82143 to the HP41 (the latter is things like the switch settings, etc). There are 3 logic signals on the 82143 (peripheral) side :
A bidirectional data line, a clock line (sourced by the peripheral) and a direction line (also sourced by the peripheral).
There is something about various 'ready' flags that you can see on the data line under certain conditions, but I would have to do some digging to find the details.
The other wires in the cable are power, ground, and IIRC a signal to say the printer exists (may not be used???).